Three distinct Staphylococcus aureus proteases have been described however little is known about their regulation by internal or external factors, or their effects on organism phenotype and virulence. We present strong evidence that these S. aureus proteases are likely to be regulated by growth conditions in the host and thereby alter exoprotein expression of the organism, significantly affecting both the phenotype and ability to cause disease, specifically in toxic shock syndrome. S. aureus proteases (I,III) have been, and protease II will be, purified using described techniques. Monoclonal antibodies to the proteases will be developed and used along with specific protease substrates and inhibitors and physical-chemical separation techniques to measure the presence of individual proteases in complex media, including infected material. Several relevant S. aureus exoproteins (Toxic Shock Syndrome Toxin-1 (TSST-1) and its precursors) have been purified and existing methods have been adapted to permit measurement of the expression of their molecular form and bioactivity. The effect of the individual proteases on each other, as well as on these other S. aureus proteins will be studied in vitro; and in vivo, using an animal model for TSS and infected patient material collected from patients with TSS and other S. aureus infections. The genotypic and phenotypic (proteases, TSST-1 and precursors) characteristics of relevant S. aureus strains will be compared in a collaborative study. It is likely that growth conditions in the host influence protease expression of S. aureus strains with resultant critical effects on exoprotein expression and virulence. Many other organisms produce proteases which may similarly affect both their phenotype and potential virulence, such that host interaction with organism proteases may have a significant influence on pathogenesis of many bacterial diseases.